Say you want to print out some values, but you don't want to call
printf - this can be for a variety of reasons, such as you're deep in
glibc or otherwise in some delicate code. You can call a bad syscall
and get the kernel to print things out:
tcbhead_t *me = THREAD_SELF;
syscall(0, me, me->stack_guard, me->pointer_guard,
me->gscope_flag);
There is no syscall == 0 (SYS_null starts at 1, to catch bugs where 0
gets passed in). The kernel kindly prints out:
[kernel] Invalid syscall 0 for proc 66
Args: 0x0000400000a13560, 0xff0a000000000000, 0xff0a000000000000,
0x0000000000000000, 0x0000000000000000, 0x00007f7fff9fece0
SW TRAP frame (partial) at 0xffffffffc8bfc790 on core 10
rbx 0x00007f7fff9feaf0
rbp 0x00007f7fff9febe0
r12 0x0000000000000000
r13 0x0000000000000000
r14 0x0000000000000000
r15 0x0000000000000001
gsbs 0x0000000000000000
fsbs 0x0000000000000000
rip 0x000040000079c7fe
rsp 0x00007f7fff9fea10
mxcsr 0x00001fa0
fpucw 0x037f
The args to the syscall are nicely printed out for you. Don't pay too
much attention to the SW TRAP frame. That's the context that actually
did the syscall(), which is a couple functions deep from where you
wrote syscall().
Barret
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